1. Field of the Invention
The present invention relates to a discharge electrode used in a laser apparatus in which a laser gas containing a halogen gas is excited by a discharge performed between electrodes, and a discharge electrode manufacturing method.
2. Description of the Related Art
FIGS. 27A and 27B are sectional views of an excimer laser apparatus or fluorine molecular laser apparatus.
In a gas laser apparatus such as an excimer laser apparatus or fluorine molecular laser apparatus, a laser gas is sealed inside a laser chamber 10. A mixed gas comprised of a rare gas (krypton Kr in the case of a KrF excimer laser, argon Ar in the case of an ArF excimer laser, and the like), a halogen gas (fluorine F2 or the like) and a buffer gas (neon Ne) is used as the laser gas. In order to cause laser oscillation by exciting this laser gas, main discharge electrodes (cathode and anode) 2 and 3 which are disposed facing each other across the optical axis L of the laser light, and preparatory dissociation electrodes 41 and 42 which cause preparatory dissociation of the laser gas between the main discharge electrodes 3 and 3 in order to facilitate the generation of a discharge between the main discharge electrodes 2 and 3, are disposed inside the laser chamber 10.
In the case of such a gas laser apparatus, as a result of the preparatory dissociation of the laser gas between the main discharge electrodes 2 and 3 by the preparatory dissociation electrodes 41 and 42, a discharge is generated between these main discharge electrodes 2 and 3, and the laser gas is thus excited so that a laser oscillation is generated. Furthermore, as is universally known, the generation of a stable discharged between the main discharge electrodes 2 and 3 results in a stable laser oscillation, so that a stable laser output is obtained.
The main discharge electrodes used in such a gas laser apparatus include the electrodes described in Japanese Utility Model Application Laid-Open No. 61-174764 (hereafter referred to as xe2x80x9cReference 1xe2x80x9d), Japanese Patent Application Laid-Open No. 62-199078 (hereafter referred to as xe2x80x9cReference 2xe2x80x9d), and Japanese Patent Application Laid-Open No. 63-227069 (hereafter referred to as xe2x80x9cReference 3xe2x80x9d).
In the electrodes described in the abovementioned Reference 1, the side surface portions of the main discharge electrodes other than the portions of the surfaces of the electrodes where a discharge is performed (hereafter referred to as the xe2x80x9cdischarge portionsxe2x80x9d) are closely covered with an insulating material in order to prevent a discharge from occurring between the main discharge electrodes and the arc discharge electrodes (corresponding to the preparatory dissociation electrodes) even in cases where the gap between the main discharge electrodes and the arc discharge electrodes is short.
Furthermore, in the electrodes described in the abovementioned Reference 2, at least portions of the laser tube or discharge members are coated with a halogen corrosion-resistant resin layer in order to suppress problems such as corrosion of the inside walls of the laser tube or discharge members, deterioration of the sealed gas and the like caused by the generation of strong ultraviolet radiation, ions, electrons and the like in large quantities in the vicinity of the main discharge electrodes.
Furthermore, in the electrodes described in the abovementioned Reference 3, insulators are mounted on the end portions, e.g., the local surface portions of the end portions, of the main discharge electrodes so that a stable glow discharge is obtained at the flat surface portions located in the centers of the main discharge electrodes, and so that insulation breakdown and arcing in the end portions of the main discharge electrodes are prevented.
Furthermore, main discharge electrodes other than the electrodes described in these references include main discharge electrodes in which the cathode surfaces in the main discharge electrodes constructed from an anode and cathode are coated with a dielectric thin film. In such electrodes, deterioration of the anode (deformation of the electrode) caused by discharge bombardment is alleviated by utilizing the drop in the discharge initiation voltage that occurs as a result of the cathode surfaces being coated with a dielectric thin film.
However, in cases where the main discharge electrodes of the abovementioned References 1 through 3 are actually installed inside the laser chamber of an excimer laser apparatus such as a krypton-fluorine (KrF) excimer laser, argon-fluorine (ArF) excimer laser or the like, or the laser chamber of a fluorine molecular (F2) laser apparatus, a reaction occurs between the halogen gas (fluorine or the like) contained in the laser gas and the discharge part of the anode as a result of the repetition of the laser oscillation operation, so that the anode is halogenated (fluorinated in the case of fluorine). Furthermore, as a result of discharge bombardment and heat, the discharge part of the anode is deformed from a flat state into a shape with indentations and projections. Moreover, the discharge part of the cathode is also halogenated, although to a slight degree compared to the anode, and the discharge part of the cathode also deteriorates as a result of sputtering.
As a result, the discharge between the main discharge electrodes becomes unstable, so that the output energy of the laser drops, thus making it impossible to obtain the desired laser output characteristics.
In order to counter this problem, it is necessary to take measures such as raising the gas pressure inside the laser chamber, raising the voltage that is applied across the main discharge electrodes, or the like. In some cases, it may be necessary to replace the deteriorated main discharge electrodes. Furthermore, even in cases where the deteriorated electrodes are replaced with fresh electrodes, problems similar to those described above still occur; as a result, there is a frequent electrode replacement cycle. When the electrodes are replaced, the entire laser chamber must be re-assembled, so that the working characteristics are extremely poor; this results in the problem of increased maintenance costs.
Even in the case of main discharge electrodes in which the cathode surface is coated with a dielectric thin film, the discharge part of the anode where a discharge is performed between the anode and cathode is not coated with such a thin film. Accordingly, as was described above, stable laser output characteristics cannot be obtained because of degeneration of the electrodes (anode) caused by halogenation (e.g., fluorination) of the electrode material, and deterioration of the anode (deformation of the electrode) caused by discharge bombardment.
Thus, if the abovementioned conventional techniques are used, a metal fluoride film is formed on the surfaces of the main discharge electrodes in proportion to the number of laser shots, so that there is an effect on the stability of the main discharge. If the stability of the main discharge reaches a range that is not permitted from the standpoint of laser performance, the electrodes must be replaced.
In regard to this problem, W/O 01/97344, U.S. Patent Application Laid-Open No. 2001/50939 (hereafter referred to as xe2x80x9cReference 4xe2x80x9d) indicates the following:
The metal fluoride film (copper fluoride in the case of copper electrodes) that is formed on the electrode discharge parts increases in proportion to the number of laser shots, and when this film reaches approximately 50 to 80% of the electrode discharge part regions, this is viewed as being the useful life of the electrodes. In cases where the main discharge is performed in regions that are covered by a metal fluoride film, the current flows through small holes with a diameter of 50 to 100 xcexcm. In regions where a metal fluoride film is formed, there is no further progression of erosion caused by fluorine. However, as the regions that are covered by a metal fluoride film in the discharge parts become smaller, the rate of erosion in the regions that are not covered by a metal fluoride film is accelerated. During the main discharge, the manner in which the current flows varies between areas in which a metal fluoride film is formed and area in which such a film is not formed. Accordingly, as the formation of a metal fluoride film progresses, the uniformity of the main discharge is lost. As a result, the beam quality suffers, and the useful life of the electrodes is reached.
As a countermeasure against this problem, according to the technique described in Reference 4, a groove that matches the electrode width is formed in the discharge part, and a metal called an xe2x80x9cerosion padxe2x80x9d that differs from the electrode matrix material is embedded in this groove. Furthermore, an annealing treatment is performed prior to this embedding in order to achieve tightly packed crystal grain boundaries in the erosion pad. As a result, it is thought that chemical erosion of the electrodes by fluorine can be prevented. In other words, in Reference 4, it is considered that the metal fluoride film that is formed on the discharge parts of the electrodes is a harmful substances that destroys the stability of the discharge, so that it is necessary to prevent the formation of such a metal fluoride film on the discharge parts of the electrodes as far as this is possible.
In the technique described in FIG. 4, since a metal fluoride film that is formed non-uniformly in the longitudinal direction of the electrodes causes a drop in the laser performance, measures are taken in order to prevent the formation of a metal fluoride film.
Here, the principle of the fluorination of metals will be described.
Fluorine is a substance that has an extremely high reactivity, and will react with various substances. When a metal is exposed to a fluorine atmosphere, the surface of the metal is eroded by the fluorine so that a metal fluoride is formed. However, in the case of a metal fluoride, the phenomenon described below progresses, so that after a fixed period of time has elapsed, the erosion of the metal by the fluorine either stops or becomes saturated at a low erosion rate.
When a metal is exposed to fluorine gas, the fluorine invades the metal to a certain depth from the metal surface. The reason for this is that even though a metal appears to be dense, there are gaps between the atoms of the metal, so that fluorine enters the metal via these gaps. Accordingly, fluorine not only reacts with the metal at the surface, but also erodes the metal by reacting with the metal at a certain depth. The erosion depth of fluorine can be expressed by an exponential function (1) that depends on the metal temperature or fluorine gas temperature.
xcex4=A exp (xe2x88x92xcex1/t)xe2x80x83xe2x80x83(1)
(xcex4: fluorine invasion depth, t: temperature of gas or metal, A and xcex1: constants determined by the types of gas and metal involved, the crystal structure, the size of atoms or molecules, and the like).
As the reaction proceeds to a certain extent, fluorination of the metal progresses, so that the film thickness of the metal fluoride exceeds the fluorine invasion depth. As a result, the fluorine entering from the surface of the metal fluoride film carbon nanocoil no longer contact the metal. Consequently, it is thought that the erosion of the metal by the fluorine either stops or becomes saturated at a low erosion rate.
One example of a technique using the effects of fluorine erosion on metals is a fluorination passivation treatment. This treatment is a treatment that modifies the surfaces of metal instruments that are used in a fluorine atmosphere, so that these surfaces are endowed with fluorine resistance. In a fluorination passivation treatment, the instrument that is being treated is exposed to fluorine gas that has been elevated to a high temperature, so that a thick metal fluoride film is formed on the surface of the instrument. In a fluorine gas atmosphere at a temperature that is lower than the temperature used during the fluorination passivation treatment, the fluorine does not have an invasion depth that exceeds the invasion depth at the time of the fluorination passivation treatment. Accordingly, the instrument is protected from fluorine erosion by the metal fluoride film that is formed by the fluorination passivation treatment.
Here, assuming that the metal is an electrode used for gas laser excitation, a case will be described in which a metal fluoride film is formed on the electrode during the operation of the laser.
This electrode used for gas laser excitation is exposed to fluorine contained in the laser gas for a long period of time inside the laser chamber. Accordingly, as in the case of the abovementioned fluorine erosion of metals in general, the electrode discharge part is eroded by the fluorine so that a metal fluoride is formed. Generally, as is mentioned in Reference 4, it appears that the following phenomena A through D occur as fluorination proceeds in the electrode discharge part, thus determining the useful life of the laser electrodes.
A. The electrical conductivity of the electrodes drops, so that the oscillation efficiency drops due to energy consumption other than that of the main discharge.
B. The distance between facing electrodes increases as a result of consumption of the electrodes, so that matching of the electrodes and power supply is lost, thus resulting in a drop in the energy transmission efficiency.
C. The shape of the discharges parts becomes rough as a result of consumption of the electrodes, so that the main discharge becomes non-uniform, and the laser excitation efficiency drops.
D. The electrodes are consumed in a non-uniform manner in the longitudinal direction, so that the main discharge becomes non-uniform, and the laser gain has a non-uniform distribution.
As a result of the abovementioned A through D, the laser performance gradually drops. At the point in time at which the drop in performance exceeds a permissible range, it is judged that the end of the useful life of the electrodes has been reached, so that maintenance work such as electrode replacement or the like is necessary.
When electrodes actually used in laser operations are analyzed, the characteristics shown in 1 through 8 below are seen:
1. A metal fluoride film is formed on the discharge parts of the electrodes in accordance with the number of laser shots.
2. In some cases, the metal fluoride film is formed uniformly in the longitudinal direction of the electrodes, while in other cases, the metal fluoride film is formed non-uniformly in the form of streaks or islands.
3. In portions where a thick metal fluoride film is formed, consumption of the electrode is inhibited.
4. The thickness of the metal fluoride film is formed at a rate of approximately 0.05 to 0.1 mm/1xc3x97109 shots. However, as the number of shots increases, the formation rate becomes saturated so that this rate approaches zero.
5. The metal fluoride film that is formed is not dense; voids are present in the interior of the film. Furthermore, metal that has not been fluorinated is dispersed in the interior of the film.
6. The shape of the discharge parts of the electrodes reaches a state in which the original shape is not preserved as a result of damage caused by fluorine and the main discharge.
7. The metal fluoride film shows complete insulating properties against a weak DC voltage.
8. The metal fluoride film is an insulator; however, even if a metal fluoride film that has the abovementioned properties is present on the discharge parts of the electrodes to a thickness of approximately 2 mm, a discharge that excites the laser is possible.
Thus, a metal fluoride film that differs from the metal fluoride film that is formed on the surface of a metal by the abovementioned fluorination passivation treatment is formed on an electrode that is eroded by fluorine as a result of use in a gas laser. The apparent reason for this is that an electrode that is used in the excitation of a gas laser is placed in an environment such as that described in (1) and (2) below, and this environment differs conspicuously from the environment in which a fluorination passivation treatment is performed.
(1) Different Heat Application Phenomena
In electrodes used in gas lasers, the temperature of the discharge part is an extremely high temperature of 100xc2x0 C. or greater. Accordingly, the discharge part is in a state that is especially susceptible to the invasion of chlorine into the interior portions of the electrode. Furthermore, gas lasers are generally operated using a pulsed power discharge system in which an electric field is intermittently applied, so that a cycle in which heat is applied over a very short time of several hundred nanoseconds, and this heat is diffused over a time period ranging from several hundred microseconds to several milliseconds, is repeated.
(2) Non-Uniformity in the Longitudinal Direction of the Electrode
Since a long laser excitation region is required in gas lasers, long discharge electrodes with a rail gap configuration are usually used. In cases where the main discharge is performed using such electrodes, even though it may appear to the eye that a glow discharge that is uniform in the longitudinal direction is generated, non-uniformity occurs in the discharge density, i.e., the current density. When non-uniformity occurs in the current density, non-uniformity is generated in the temperature of the discharge parts. Non-uniformity in the temperature of the discharge parts has a great effect on the properties of the metal fluoride film that is formed on the discharge parts. In areas where the temperature is high, a thick metal fluoride film is formed, and the electrode matrix material is greatly eroded. Conversely, in areas where the temperature is low, a thin metal fluoride film is formed, and the electrode matrix material is not greatly eroded.
In areas where the temperature of the discharge parts in the electrodes is high so that these parts are subjected to fluorine erosion, a thick metal fluoride film or fluorinated layer is formed. The metal fluoride film is an insulator, and has a high electrical resistance. Accordingly, in cases where a current flows through the metal fluoride film, the metal fluoride film generates heat. Thus, in the discharge parts where a metal fluoride film is formed, the temperature rises so that fluorination proceeds to an increasing extent. Furthermore, this metal fluoride film generally has a low density. As a result, the fluorinate portions swell in the discharge part, so that these portions protrude compared to the portions that are not fluorinated. These protruding portions function as xe2x80x9clightning rodsxe2x80x9d, so that the current arising from the main discharge is concentrated. Accordingly, the temperature of the fluorinated discharge parts rises even further locally.
When the temperature of the discharge parts shows such local rises, the invasion of the electrode matrix material by the fluorine is facilitated. As a result, fluorination proceeds not only from the discharge parts of the electrodes, but also from the interior portions of the electrodes. When fluorination proceeds from the interior portions of the electrodes, the electrode matrix material and the metal fluoride film formed on the discharge parts are joined in a graded manner resembling a gradient metal alloy (an alloy in which the proportion of the insulator is greater than that of the conductor at portions closer to the surface, while the proportion of the conductor is greater than that of the insulator at portions further inside thereof), so that an extremely strong structure is created. In such a state, the metal fluoride film that is formed remains on the discharge parts without peeling from the discharge parts. Furthermore, in portions where a thick metal fluoride film or metal fluoride layer is formed, it becomes difficult for fluorine or fluorine radicals to invade from the surface, so that the rate of erosion of the electrode matrix material by the fluorine becomes more gradual. As a result, the electrode consumption rate drops.
Conversely, in areas where the temperature of the discharge parts in the electrodes is low, so that the fluorine erosion is not that great, a thin metal fluoride film is formed on the surface layer. In areas where the temperature is thus low, the fluorine cannot invade deeply into interior of the electrode. As a result, fluorination of the electrode occurs in the vicinity of the surface. The bonding force of the metal fluoride film that is thus formed with the bulk material of the electrode is weak, so that the film is brittle. Accordingly, the film is easily stripped by sound waves generated by the main discharge, vibrations from the motor, the flow of gas, bombardment by the main discharge current, and bombardment by electrons, ions, neutral particles and excited particles, etc. Such a reaction is endlessly repeated; as a result, the electrode matrix material is greatly eroded.
Thus, the metal fluoride film that is formed on the electrodes during the operation of a laser differs from the metal fluoride film that is formed by a fluorination passivation treatment in that (1) the film is formed in a non-uniform manner, and (2) there are portions that are susceptible to fluorine erosion and portions that are resistant to fluorine erosion.
The abovementioned phenomenon A and the abovementioned characteristic 8 conflict with each other. This can be explained by the abovementioned characteristic 5.
Generally, in the case of a pulsed power discharge, the voltage is applied for a short period of time; accordingly, the discharge has high-frequency characteristics. As a result of the surface skin effect, a high-frequency current has the property of flowing through the surface layer (several hundred xcexcm) of a substance. In other words, a high-frequency current is not greatly affected by the electrical conductivity of the substance. Accordingly, this high-frequency current differs from a direct current in that this current can flow even at the surface of an insulator. As in the case of a high-frequency current, the relative ease with which a pulsed power discharge current flows depends not on the electrical conductivity of the substance itself, but rather on the size of the surface area of surfaces such as the interfacial surfaces between substances.
This will be examined in terms of a metal fluoride film. Voids and metal portions that have not bee fluorinated are present in a dispersed state in the metal fluoride film formed on the discharge parts of the electrodes by a laser operation. Current pathways from the surface of the metal fluoride film to the electrode matrix material are maintained by the surfaces of voids and the interfacial surfaces between fluorinated metal areas and non-fluorinated metal areas. Thus, electrical conductivity with respect to the pulsed power discharge current is ensured. Accordingly, the metal fluoride film formed by an actual laser operation does not show high insulating properties with respect to the pulsed power discharge. Consequently, the drop in the laser performance caused by the presence of a metal fluoride film is not that great.
Thus, it is confirmed that the metal fluoride film that was generally thought to cause a drop in the laser performance is effective in decreasing electrode consumption without causing a great drop in the laser performance. In cases where the laser is operated in a state in which the gas pressure, fluorine concentration, input energy, applied voltage, electrode temperature and the like are optimized in order to maintain the laser performance, such a metal fluoride film is formed on the electrode discharge parts.
However, since the laser operation is not performed for the purpose of forming a metal fluoride film on the electrode discharge parts, the properties of the metal fluoride film that is formed cannot be regulated. As a result, the metal fluoride film tends not to be formed uniformly on the electrode discharge parts. A non-uniform metal fluoride film results in non-uniform consumption of the electrodes, so that the laser output becomes unstable. Furthermore, it becomes difficult to predict the useful life of the electrodes. Moreover, the shape of the electrode discharge parts is broken down, so that the laser oscillation efficiency drops.
Accordingly, it is an object of the present invention to provide a discharge electrode and discharge electrode manufacturing method which make it possible to inhibit deterioration of the electrodes so that a stable laser output is obtained, and which make it possible to prolong the intervals at which the electrodes must be replaced.
Accordingly, the first invention is a discharge electrode which contains a metal material, and which is used in a laser apparatus in which a laser gas containing a halogen gas is excited by a discharge performed between electrodes, wherein a film containing one or more substances that have a higher hardness than the metal and that are less reactive with a halogen gas than the metal is formed on surfaces of portions where the discharge is performed.
Furthermore, the second invention is a discharge electrode which contains a metal material, and which is used in a laser apparatus in which a laser gas containing a halogen gas is excited by a discharge performed between electrodes, wherein a film containing one or more substances that have a higher melting point than the metal and that are less reactive with a halogen gas than the metal is formed on surfaces of portions where the discharge is performed.
As is shown in FIGS. 1A and 1B, a film 5 is formed on the discharge parts 2a and 3a of the main discharge electrodes 2 and 3. In order to prevent the erosion of the discharge parts 2a and 3a by the halogen gas, one or more substances that tend not to react with the halogen gas contained in the laser gas, i.e., one or more halogen-resistant substances, are contained in the film 5. Furthermore, in order to prevent deformation of the discharge parts 2a and 3a by the bombardment or heat of the main discharge, one or more substances that have a higher hardness than the metal of the main discharge electrodes 2 and 3 or one or more substances that have a higher melting point than the metal of the main discharge electrodes 2 and 3 are contained in the film 5.
In the first and second inventions, a main discharge is performed between the main discharge electrodes 2 and 3 in which the abovementioned film 5 is formed on the discharge parts 2a and 3a. In this case, the erosion, bombardment and heat arising from the halogen gas accompanying the main discharge are absorbed by the film 5; accordingly, the discharge parts 2a and 3a are unaffected. Consequently, deterioration of the main discharge electrodes 2 and 3 is inhibited, so that a stable laser output can be obtained. Furthermore, since the useful life of the main discharge electrodes 2 and 3 is prolonged, the intervals at which the main discharge electrodes 2 and 3 must be replaced can be extended. Accordingly, maintenance costs can be reduced.
The third invention is according to the first invention, wherein the abovementioned film comprises an insulator.
The fourth invention is according to the second invention, wherein the abovementioned film comprises an insulator.
As shown in FIG. 2, an insulator 51 is used in the film 5 that is formed on the discharge parts 2a and 3a of the main discharge electrodes 2 and 3.
In the third and fourth inventions, the main discharge is performed through the insulator 51; as a result, the impact itself of the main discharge can be reduced and the consumption of the insulator 51 itself can be inhibited. Accordingly, the main discharge electrodes 2 and 3 have a long useful life.
The fifth invention is according to the first invention, wherein the abovementioned film comprises a mixture of an insulator and a conductor.
The sixth invention is according to the second invention, wherein the abovementioned film comprises a mixture of an insulator and a conductor.
As is shown in FIGS. 3A through 3C, a mixture 52 is used in the film 5 that is formed on the discharge parts 2a and 3a of the main discharge electrodes 2 and 3. The mixture ratio of the insulator and conductor is uniform in this mixture 52.
In the fifth and sixth inventions, a conductor is mixed with the mixture 52; as a result, the resistance between the main discharge electrodes 2 and 3 is lowered. Accordingly, the laser output is increased.
The seventh invention is according to the first invention, wherein the abovementioned film is a laminated film in which layers of the abovementioned conductor and layers of the abovementioned insulator are alternately laminated.
The eighth invention is according to the second invention, wherein the abovementioned film is a laminated film in which layers of the abovementioned conductor and layers of the abovementioned insulator are alternately laminated.
As is shown in FIGS. 4A and 4B, a mixture 53 is used in the film 5 that is formed on the discharge parts 2a and 3a of the main discharge electrodes 2 and 3. Layers of an insulator and layers of a conductor are alternately laminated in the mixture 53.
In the seventh and eighth inventions, when a main discharge is repeated between the main discharge electrodes 2 and 3, the mixture 53 itself is consumed. This consumption of the mixture 53 occurs as a result of the layers exposed at the surface of the mixture 53 being stripped from the layers underneath. Accordingly, as a result of the consumption of the mixture 53, the layers of a conductor and the layers of an insulator are alternately exposed at the surface of the mixture 53. When a conductor layer is exposed at the surface of the mixture 53, the laser output that is obtained differs from that obtained when an insulator layer is exposed at the surface of the mixture 53. Accordingly, if the number of laminated layers is known beforehand, the useful life of the mixture 53 itself can be ascertained by detecting variations in the laser output.
The ninth invention is according to the third invention, wherein the abovementioned insulator is a ceramic.
The tenth invention is according to the fourth invention, wherein the abovementioned insulator is a ceramic.
The eleventh invention is according to the fifth invention, wherein the abovementioned insulator is a ceramic.
The twelfth invention is according to the sixth invention, wherein the abovementioned insulator is a ceramic.
The thirteenth invention is according to the seventh invention, wherein the abovementioned insulator is a ceramic.
The fourteenth invention is according to the eighth invention, wherein the abovementioned insulator is a ceramic.
The ninth through fourteenth inventions indicate the concrete substances of the third through eighth inventions.
Here, (1) the most ideal electrodes, and (2) the next most ideal electrodes, will be described on the basis of facts ascertained by verification testing.
(1) Most Ideal Electrodes
As was described above, the laser performance drops as a result of damage to the electrodes. Accordingly, in order to maintain the laser performance, it is most ideal to devise the system so that the electrodes are not damaged. The greatest cause of damage to the electrodes is the main discharge that is performed in a fluorine atmosphere, i.e., a fluorine discharge. Accordingly, it is universally known that the use of a substance that has a high resistance to a fluorine discharge as the electrode matrix material is effective in suppressing electrode damage.
However, from the standpoint of the electrical characteristics of the electrodes, materials that have a high resistance to a fluorine discharge are not suitable as electrode matrix materials. Generally, materials that have a high fluorine discharge resistance are insulators, so that the electrical conductivity required in an electrode cannot be realized.
However, in the case of electrodes whose structure has been optimized as described above, electrical conductivity with respect to the pulsed power discharge current is ensured, and the electrodes also possess fluorine discharge resistance. In the case of such electrodes, most of the electrode is formed from a substance (conductor (metal)) that is suitable for an electrode, with only the electrode discharge parts that are exposed to the fluorine discharge being covered by a substance (insulator) that possesses fluorine discharge resistance.
As a result of the realization of such a structure, the electrode discharge parts are not eroded by fluorine, and the performance of the electrode discharge parts as electrodes is ensured; accordingly, the main discharge that is required for laser excitation can be formed. Since the electrodes are not eroded by fluorine, fluctuation in the laser performance is eliminated from the outset, so that maintenance is unnecessary until the design useful life of the electrodes is ended.
(2) Next Most Ideal Electrodes
In the case of electrodes in which a substance that has fluorine discharge resistance is formed on the electrode discharge parts as in (1) above, damage is inhibited. However, in cases where a substance that has fluorine discharge resistance is formed on the electrode discharge parts, the following problems are encountered:
Even in the case of a substance with extremely superior fluorine discharge resistance, there may be instances of local damage caused by the fluorine discharge. When such local damage occurs, the discharge is concentrated in the damaged areas, so that the uniformity of the discharge is lost. Furthermore, this leads to non-uniformity of the electrode erosion. Accordingly, the next most ideal approach is to ensure a path for the passage of the pulsed power discharge beforehand.
In the case of such electrodes, most of the electrode is formed from a substance (conductor (metal)) that is suitable for an electrode, with the electrode discharge parts that are exposed to the fluorine discharge being covered by a film of a mixed material in which a substance (insulator) that possesses fluorine discharge resistance and a substance (conductor (metal)) that is suitable for an electrode are uniformly dispersed. As was described above, a pulsed power discharge has the property of flowing through the surface layer of a substance. Accordingly, the pulsed power current reaches the electrodes via the interfacial surfaces between the insulator and conductor (metal).
In the case of such a structure, the metal in the mixed material and the electrode matrix material itself are fluorinated. The conditions of formation of this fluoride are as follows:
First, the metal portions constituting the conductor in the film of mixed material that covers the electrode discharge parts are fluorinated so that a metal fluoride is formed. An insulator that possesses fluorine discharge resistance is present around this metal fluoride. As a result of the presence of this insulator, the metal fluoride remains without being stripped from the electrode discharge parts. Furthermore, even though the insulator that possesses fluorine discharge resistance may be slightly stripped from the electrode discharge parts, the metal fluoride and metal both form a film of a mixed material. Generally, since metal fluorides have a good fluorine resistance, this mixed-material film possesses good fluorine discharge resistance. Furthermore, since a film with such a composition has interfacial surfaces between different substances, sufficient electrical conductivity is shown with respect to the pulsed power current as a result of the surface skin effect. As a result of these properties, even if there is some fluctuation in the composition of the substance constituting the electrode discharge parts, the shape of the outermost surfaces of the electrodes and the electrical conductivity of the electrodes show no great fluctuation compared to the fluctuation that occurs prior to the discharge.
As the laser operation continues so that the invasion of fluorine moves beyond the mixed-material film and reaches the electrode matrix material (metal), the fluorine begins to invade the electrode matrix material. However, the composition of the fluoride that is formed in this case produces a structure in which the metal and metal fluoride are uniformly dispersed. Furthermore, numerous voids are present in this fluoride. Specifically, as was described above, a layer that does not impede the pulsed power discharge current is formed. Furthermore, since a mixed-material film is present on the discharge parts of the electrodes, there is no stripping of the metal fluoride even if the electrodes are fluorinated.
As a result of the progress of the abovementioned phenomena, the substance formed as a coating film on the electrode discharge parts and the electrode matrix material itself are gradually fluorinated. However, this case differs from the universally known case in the electrodes with the following properties can be realized:
1. There is almost no fluctuation in the shape of the outermost surfaces of the electrodes.
2. There is almost no consumption of the electrodes, so that the distance between the electrodes shows almost no fluctuation.
3. The electrode discharge parts are modified uniformly in the longitudinal direction of the electrodes.